The present invention relates to packaging of optical fibers to form cables for use in transmission of optical signals. It further relates to, but is not limited to, cables containing a number of optical fibers each used as a channel for signal transmission in a given direction.
The primary consideration for such cables is that increases in optical signal attenuation, due to distortion of fiber axis, as a consequence of packaging, should be minimized and preferably eliminated. Other considerations include appropriate mechanical properties for specific types of installations and ease of access to fibers for termination and splicing.
The present invention is a construction of a cable utilizing tapes of multiple fibers as a sub-assembly. However, the construction improves on the concept of wrapping over a core and recognizes the necessity for incorporating pairs in the cable for repeater power feed, order and fault location. It also recognizes the necessity of pressurization from installed cable maintenance consideration in the absence of suitable fault-locating devices.
One embodiment of this invention has an extruded plastic core with one or more, preferably at least two, helical grooves. The core should preferably (but may not) have a central strength member in the core for axial strength. One or more tapes may be placed in each groove. Metallic pairs may also be placed in a groove.
To prevent the optical fiber tapes or pairs from coming out of the grooves, the cable core may be bound with binders, tapes with or without longitudinal core wraps. Appropriate sheaths may be applied on the cable core for completing the cable. The sheath may incorporate a strength member to provide axial strength as well. It should also provide necessary compressive or impact strength and should be suitable for pressurization when needed.
One such sheath may be a continuously welded metal tube followed by a polyethylene jacket. Corrugated steel longitudinally folded with overlap and flooded with asphaltic compound may be applied over the inner jacket, followed by a polyethylene outer jacket.
Advantages of placing tapes in a helical groove, with sizable depth, over wrapping on a core will be pointed out in describing the structure shown in the drawing.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.